s390/pci: do more bus setup in zpci_bus_scan()

Pull setting the maximum bus speed and multifunction attribute into
zpci_bus_scan() in preparation for handling bus creation separately
from scanning the bus.

Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Acked-by: Pierre Morel <pmorel@linux.ibm.com>
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>